1. Field of the Invention
The present invention relates to radio base stations and scheduling methods, and more particularly, to a radio base station and a scheduling method for controlling packet assignment according to the quality of radio communication in a digital radio communication system.
2. Description of the Related Art
An adaptive rate method may be used in a digital radio communication system as a method for changing the communication speed according to the quality of a radio channel in order to increase the efficiency of frequency use. An upstream communication rate, a downstream communication rate, or both are determined according to the quality of the radio channel.
In a system that uses an adaptive rate method where an upstream rate and a downstream rate are determined according to the quality of a channel, such as a 1xEV-DO system, a radio terminal determines the downstream rate according to the quality of a downstream channel measured by the radio terminal.
FIG. 11 shows the relationship between downstream channel quality and determined downstream channel rates in a conventional adaptive rate method.
In general, the lower the channel quality is, the larger the number of slots used per radio packet; and the higher the channel quality is, the smaller the number of slots used. When a hybrid ARQ method is used as a retransmission method for downstream signal data, if packet recovery is found to be possible at an intermediate slot among assigned slots, slot assignment is cancelled at the slot and the remaining slots can be assigned to another user. With the use of this method, flexible channel assignment is possible according to the quality of the downstream channel.    Non-patent Document 1: 9.3.1.3.1 Modulation Parameters (pages 9-67 to 9-70), cdma2000 High Rate Packet Data Air Interface Specification, C.S0024-0, V 4.0, The Third Generation Partnership Project 2 (3GPP2) Specifications, URL: http://www.3gpp2.org/Public_html/specs/tsgc.cfm, searched for on Sep. 11, 2007